Contemporary design processes often require the rapid fabrication of prototypes and models of complex mechanical parts in low volumes and with minimum setup and fabrication times to allow the evaluation and testing of the design of such parts within very short design and development periods. Most conventional fabrication methods, however, are unsuitable for such purposes. Manual machining, for example, is sometimes suitable for relatively simple designs but is too slow and expensive for complex designs and Computer Numerically Controlled (CNC) machine processes, while suitable for complex designs, have significant limitations as regards the types or configurations of parts that can be fabricated.
The need for rapid, low cost, low volume fabrication of complex parts has generally been met by the development of various three dimensional (3D) modeling processes that employ layer by layer “printing” processes. In typical 3D modeling processes of the prior art, a part is built up by the successive deposition of layers comprised of a “model” material forming the actual part and a sacrificial material that provides support for the model material during the process, with the sacrificial material being subsequently removed to leave the model material forming the actual part.
Typical examples of 3D modeling processes of the prior art include, for example, Householder, U.S. Pat. No. 4,247,508, which describes a modeling process that employs two substances, one a fill material and the other a mold material, that are deposited layer by layer to build an article. The two materials in each layer are not in contact with each other while the layer is being formed because Householder '508 uses a grid to separate the two materials as they are being deposited. After the materials in each layer are deposited, the grid is moved to the next layer so that the two materials may fill the space left by the removed grid and thereafter solidify in contact with each other in the same layer.
Helinski, U.S. Pat. No. 5,136,515, describes a method wherein a three dimensional model is produced layer by layer by jetting droplets of at least two solidifiable materials, one material forming the article and a second material forming a support for the article. The second material is subsequently removed by heating, cutting, melting, chemical reacting, and so on, to leave the desired article.
Penn, U.S. Pat. No. 5,260,009, describes a system and process for making three dimensional objects by dispensing layer upon layer of modeling material using an inkjet which is turned “on” or “off” according to a two dimensional data map of each layer of the object. The two dimensional data map is stored and relayed by a microprocessor and defines locations on a matrix at which printing is to occur in a manner such as is used in printing images using raster scan printing.
Sanders, Jr. et al., U.S. Pat. No. 5,506,607, describes a system for building three dimensional models by vector plotting layer-upon-layer applications of solidifiable substances. The layers are formed by expelling minuscule beads of the substances in a liquid or flowable phase onto a platform from one or more jets wherein the jets and platform are relatively movable in the X, Y and Z coordinate system and the beads are deposited along vectors during X-Y relative movement.
Sanders, Jr. et al., U.S. Pat. No. 5,740,051, describes a method and apparatus for producing a 3-D model by forming a continuous plurality of parallel layers of modeling material by repeatedly producing a plurality of bead producing drops of the modeling material for deposition at desired locations, controlling the locations and timing of deposition to produce vectors in any and all directions required to produce an outer surface defining a wall of a layer with a desired surface finish, and adjusting the distance of the location of drop production to the location of drop deposition in preparation for the formation of a subsequent layer.
Penn et al., U.S. Pat. No. 6,175,422, describes a method and process for computer-controlled manufacture of three dimensional objects by dispensing a layer of a first insoluble material, such as a liquid, onto a platform at predetermined locations corresponding to a cross-section of the object, which then hardens. A second material, preferably water soluble, is then sprayed onto this layer to thereby encapsulate the hardened insoluble material. The uppermost surface of this encapsulant is planed, thus removing a portion of the encapsulant to expose the underlying insoluble material for a new pattern deposition. After the resulting planing residue is removed, another layer of liquid, insoluble material is dispensed onto the planed surface. The insoluble material can be of any color and may vary from layer to layer, and from location within a layer to location with a layer. These steps are repeated, until the desired three dimensional object, encapsulted in the soluble material, is completed. At this point, the object is either heated or immersed in solvent, thereby dissolving the soluble material and leaving the three dimensional object intact.
In typical embodiments of the 3D modeling processes, therefore, examples of which have been described above, the modeling and sacrificial materials are comprised of two materials having differing mechanical and/or chemical characteristics with the differences between the modeling and sacrificial materials being such that the sacrificial material can be selectively removed after the fabrication is completed. For example, in some implementations the sacrificial material may have a lower melting temperature than the modeling material or may be dissolvable by a solvent that does not effect the modeling material. Less common implementations of 3D modeling processes, which are not pertinent to the present invention, may construct the part and its model sacrificial regions from a single material having two different physical states or phases, depending, for example, upon whether a given region has been radiated by a specific type of laser radiation or has been treated with a binding agent or solvent, thereby converting treated and untreated regions of the material into the equivalent of modeling and sacrificial material.
The layers are typically laid down one layer at a time and one region or line of material at a time by drop-by-drop deposition of the materials on a previous layer or base by corresponding drop-on-demand print heads generally similar to those used in ink jet printers. Each layer is then planed to a level, uniform surface upon which the next layer may be deposited.
A system using two materials, that is, a model material and a sacrificial material, will thereby require two drop-on-demand print heads, the position and motion of each head which must be controlled according to the intended point of deposit of each drop of material. It must also be noted that each drop of sacrificial or model material is ejected from the corresponding drop-on-demand print head in a molten or liquid or semi-liquid state and solidifies only after it is deposited as part of the layer presently being laid down. This process is fundamental to the operation of drop-on-demand type print heads and additionally allows each drop to deform and to adhere to the previously deposited and solidified drops, including those of the previously deposited layer, before hardening.
The depositing of the drops of sacrificial or model material in a liquid or semi-liquid state, however, requires that each drop be deposited onto a supporting surface, typically the previously deposited layer. This, in turn, requires that each layer extend at least the maximum horizontal extent of the part above that layer, including those areas of a layer that lie under any overhanging or undercut regions of a part, although such temporary supporting areas of the layers may be subsequently removed when the modeling process is completed.
It will therefore be apparent that the 3D modeling systems of the prior art suffer from a number of inherent disadvantages. For example, the necessity that each layer must be fully supported by a lower layer requires that each layer provide a platform or support for the entire maximum horizontal model and support dimensions of the layer above it. In a typical part, however, much of the deposited material is thereby merely sacrificial material that must be subsequently removed, so that much of the deposited material is effectively “waste”. In addition, the depositing of the model and sacrificial material on a drop-by-drop basis is very time consuming, particularly when a significant proportion of the material is subsequently sacrificed, or wasted, in order to obtain the final part.
In addition, drop-by-drop deposition requires the use of print heads having relatively small jetting orifices, which limit the rate at which material can be deposited, limit the types of material that can be deposited to those materials capable of being ejected as drops through a small orifice, and reduce the print head reliability because the small orifices are more readily subject to blockage.
In addition, a typical 3D modeling process according to the prior art will lay down a layer by first depositing the model material, that is, constructing a one layer thick section of the part itself, and then filling in the remainder of the layer area with the sacrificial support material, so that the sacrificial support material functions only as a support for the next layer. This sequence of deposition, however, means that the dimensions, the texture and the quality of the finished part is determined solely by the qualities and the characteristics of the model material which are, in turn, largely determined by the characteristics required to form an initially free-standing structural element. The result, however, is to limit the characteristics of the model material in a way that is determined more by the modeling process than by the desired final characteristics of the finished part, so that the desired material and finish characteristics of the finished part often cannot be satisfactorily achieved.
The present invention addresses these and other related problems of the prior art.